Oilpan: Prepare to move RenderObject and RenderObjectChildList to Oilpan heap.

Source/core/rendering/RenderListMarker.h

Index: Source/core/rendering/RenderListMarker.h
diff --git a/Source/core/rendering/RenderListMarker.h b/Source/core/rendering/RenderListMarker.h
index 9e50085bf6a5b060870c3b39561e3a4d4a2d2b46..8eb262a519269a42c3837a374037089f7eb7d77c 100644
--- a/Source/core/rendering/RenderListMarker.h
+++ b/Source/core/rendering/RenderListMarker.h
@@ -38,6 +38,7 @@ public:
     static RenderListMarker* createAnonymous(RenderListItem*);
 
     virtual ~RenderListMarker();
+    virtual void trace(Visitor*) OVERRIDE;
 
     const String& text() const { return m_text; }
 
@@ -82,7 +83,7 @@ private:
 
     String m_text;
     RefPtr<StyleImage> m_image;
-    RenderListItem* m_listItem;
+    RawPtrWillBeMember<RenderListItem> m_listItem;
 };
 
 DEFINE_RENDER_OBJECT_TYPE_CASTS(RenderListMarker, isListMarker());